Optical passband filters are important components in for example wavelength division multiplexing (WDM) systems to provide various optical signal processing functions such as multi/de-multiplexing optical data, balancing the signal power, and adding/dropping channels. The practical applications of optical bandpass filters require that the filters shall have flat passband transmission, wide bandwidth, and high band isolation.
As a class of important bandpass filters, optical interleavers which fundamentally function as optical filter banks have attracted significant attention recently. Unlike the single-input-single-output bandpass filters, an optical interleaver can offer several channels of phase-shifted bandpass transmissions simultaneously at different outputs.
Several conventional techniques have been proposed for interleaver designs. These include Mach-Zehnder based interleavers, Michelson-Gires-Tournois interleavers (MGT), Birefrigent-Gires-Tournois interleavers (BGT) and compound etalon interleavers.
One common drawback of the above mentioned interleaver design schemes is that they cannot be implemented by an all-fibre structure. In optical communication systems, using all-fibre instruments can decrease the insertion and connection losses where optical devices are connected serially. In addition, all-fibre implementations can provide further advantages such as uniform response over a wide wavelength range, very low dispersion and low polarisation dependence.
Furthermore, most of the conventional interleavers proposed so far have been limited to only two ports structures. In order to achieve high channel counts for a communication system, a number of the conventional interleavers have to be cascaded. In practice, the insertion loss will increase with the number of interleaving filters being used. As a follow-on characteristic, only configurations of power-of-two are possible, and this limits the capability of such optical interleavers in multiplexing more complicated code in optical communications.
Recently, a three-port interleaver using a lattice configuration of 3×3 directional couplers was reported in “All fibre 3×3 interleaver design with flat-top passband,” IEEE P. Technol. Lett., Vol. 16, pp. 168-170, 2004, by the present inventors. The disclosed interleaver is a Finite Impulse Response (FIR) filter having only feed-forward interference paths. Three cascaded 3×3 directional couplers are used in the disclosed interleaver.
It is with the knowledge of the above mentioned prior art interleaver design characteristics that the present invention has been made and is now reduced to practice.